1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly to a multi-domain liquid crystal display device being excellent in viewing angle characteristics.
2. Description of the Related Art
A conventional Twisted Nematic (TN) type liquid crystal display device being widely used is so constructed that, as a liquid crystal molecule changes a direction of its director (orientation vector) toward an electric field in response to a voltage applied thereto, a state displaying a xe2x80x9cbrightxe2x80x9d color appearing at a time of applying no voltage, in which the liquid crystal molecule is in a twisted state being parallel to a substrate surface, is gradually changed to a state displaying a xe2x80x9cblackxe2x80x9d color. However, the conventional TN-type liquid crystal device has a problem in that its viewing angle is small due to a limitation associated with behaviors of the liquid crystal molecule activated by the applied voltage. The small viewing angle is remarkable in a rising direction of the liquid crystal molecule f or displaying gray shades in particular.
A method of improving viewing angle characteristic of the liquid crystal display device is disclosed in Japanese Patent Application Laid-open No. Hei6-43461. FIG. 47 is a partial schematic cross-sectional view showing a pixel of the conventional multi-domain liquid crystal display device disclosed in the above patent application. In the disclosed technology, viewing angle characteristic is improved by obtaining a liquid crystal cell by orientating a liquid crystal molecule 22 having negative dielectric constant anisotropy in a homeotropic direction and then putting it between two polarizers (not shown) in which polarization axes are intersected at right angles and using common electrodes 81 having an aperture section 74 to concentrate an electric field on a skew in each pixel and to cause each pixel to be divided into more than two domains, that is, multi-domains. Moreover, in this technology, the viewing angle for the black color can be improved by using an optical compensating plate as necessary. Furthermore, the viewing angle characteristic is improved in not only the liquid crystal cell obtained by homeotropic orientation of the liquid crystal but also in the liquid crystal cell obtained by TN orientation of a liquid crystal by using the common electrodes 81 having the aperture section 74, by causing a skew electric field to be generated and by dividing each pixel into more than two domains.
Another technology to try to improve a viewing angle characteristic is disclosed in Japanese Patent Application Laid-open No. Hei7-199190. In the disclosed technology, the liquid crystal domain is stabilized by providing a common electrode with an aperture section (serving as an orientation control window), by mounting an orientation control electrode in a manner that it surrounds a pixel electrode and by enhancing a skew electric field in an area surrounding the pixel.
Additionally, Japanese Patent Application Laid-open No. Hei7-230097 also discloses a technology to improve a viewing angle characteristic in which an orientation control electrode formed integrally with a gate bus line is provided on each pixel and each pixel is divided into more than two liquid crystal domains by a skew electric field generated by the orientation control electrode.
Furthermore, in technology disclosed in Japanese Patent Application Laid-open No. Hei10-20323, a pixel is provided with an aperture section and a control electrode is placed at a position of the aperture section to form a plurality of liquid crystal domains.
Also, in technology disclosed in Japanese Patent Application Laid-open No. Hei10-301114, in a liquid crystal cell obtained by orientating a liquid crystal having a negative dielectric constant in a homeotropic direction, an orientation film is provided with a protrusion and a tilt direction of the liquid crystal is controlled by the protrusion at a time of applying a voltage to divide each pixel into more than two domains.
However, the technology disclosed in the Japanese Patent Application Laid-open No. Hei6-43461,in which the common electrodes 81 are provided with the aperture section 74, has a problem in that microfabrication processes such as a xe2x80x9cphotoresist process for the common electrodexe2x80x9d or a like are required to fabricate a pixel, which are not required in the production processes of an ordinarily used mono-domain type TN liquid crystal display device, and further highly sophisticated technology of laminating upper and lower substrates is also required. This problem presents a serious problem especially in a case of fabricating an ordinary active matrix liquid crystal display device using a switching device such as a TFT (Thin Film Transistor). That is, in the ordinary active matrix liquid crystal display device, when the switching device (active device) such as the TFT is mounted on one transparent substrate (TFT substrate), the microfabrication processes including the photoresist process or the like is required only on the TFT substrate side and is not required by an electrode on an other transparent substrate (opposed substrate) side usually called a xe2x80x9ccommon electrodexe2x80x9d. However,in the conventional technology in which the common electrode is provided with the aperture section, the microfabrication processes such the photoresist process as not required in the ordinary cases is required for fabricating the above common electrode, thus causing increased numbers of processes for production of the device and highly sophisticated technology of laminating upper and lower substrates to be required.
To solve this problem, one would think of a technology in which an orientation of the liquid crystal can be controlled by mounting an aperture section or a slit on the pixel electrode formed on a TFT-formed substrate side in which a switching device such as the TFT or a like is mounted to make the pixel electrode generate a skew electric field. This is because the fabrication of the pixel electrode on the TFT substrate side originally requires a patterning process and therefore does not require additional further processes. However, it is impossible to control a liquid crystal domain in a stable manner by such methods. A reason is that, although a tilt direction of an electric field in an area surrounding the aperture section occurring when the aperture section is mounted on the common electrode as disclosed in the Japanese Patent Application No. Hei6-43461, is matched to a tilt direction of the electric field in an area surrounding the pixel electrode 71 (see FIG. 19), a tilt direction of the electric field in the area surrounding the aperture section occurring when the aperture section is mounted on the pixel electrode formed on the TFT substrate side is not matched to the tilt direction of the electric field in the area surrounding the pixel electrode.
If the control electrode is disposed in the area surrounding the picture electrode as shown in the Japanese Patent Application Laid-open No. Hei7-199190, although skewing of the electric field in the area surrounding the pixel electrode can be enhanced, since it is necessary to mount the aperture section on the common electrode in the substrate existing on the opposite side, the above problem cannot be solved.
If the control electrode is disposed on the picture electrode and a potential of the control electrode is set to an appropriate level as shown in the Japanese Patent Application Laid-open No. Hei7-230097, it is possible to generate the required skew electric field. However, since state of occurrence of the skew electric field is also changed by a change in polarity of the pixel electrode potential at a time of driving in an inverted manner when the polarity of the pixel electrode potential is inverted at a predetermined period, stable and reliable control on the liquid crystal domain is impossible. Moreover, since the control electrode is so constructed integrally with a gate bus line and the control electrode potential cannot be suitably changed in accordance with ON/OFF operations of the pixel, the electric field in a skew direction occurs even at a time of the OFF operation (time of displaying the dark state) of the pixel, which causes a leak of light in the area surrounding the control electrode, thus resulting in degraded display contrast. If a light blocking layer is provided to prevent such light leak, a great decrease in aperture rate may occur. Furthermore, in ordinary cases, because a DC voltage is applied to the common electrode through the gate bus line during a period except period for selecting the pixel, if the control electrode is constructed integrally with the gate bus line, the DC voltage continues to be applied to a liquid crystal layer in the area for displaying, thus causing the degradation of the reliability in a display device.
Even in the technology disclosed in the Japanese Patent Application Laid-open No.Hei10-20323 in which the pixel electrode is provided with the aperture section and the control electrode is placed at the position of the aperture section, since no means is provided for controlling the control electrode potential for every pixel at a time of the display operation, as in the case of the Japanese Patent Application Laid-open No. Hei7-230097, there still remains a problem in that stable and reliable control on the liquid crystal domain is impossible.
To solve these problems in the conventional technologies as disclosed in the Japanese Patent Application Laid-open Nos. Hei7-230097and Hei10-20323, a technology may be available in which the control electrode individually mounted for each pixel can be controlled by switching devices each being individually mounted for each pixel. However, in this technology, since the switching device and the drain bus line have to be individually and separately mounted corresponding to each of pixel electrodes and of control electrodes, configurations of the device is made complicated accordingly and such devices are not realistic in terms of their production cost and yield.
The technology disclosed in the Japanese Patent Application Laid-open No. Hei10-30114 in which the orientation film is provided with the protrusion has also a problem in that division of the area by using the protrusion is effective only in vicinity of the protruded area and, therefore, in order to realize the reliable division of the area, it is necessary to form the protrusion not only on the orientation film of the TFT substrate existing on one side where the switching device is formed, but also on the orientation film on the opposed substrate side, this causes great increases in the processes and further the technology cannot be realized without very exact lamination process of both substrates.
The inventors of the present invention have already disclosed a technology in Japanese Patent Application No. Hei11-180615 in which a signal voltage is applied to a control electrode directly connected to a switching device and a partial voltage of the signal voltage is applied to a pixel electrode through a coupling capacitor. According to the disclosed technology, two potentials of the control electrode and pixel electrode can be easily controlled by one switching device, which allows an orientation state of a liquid crystal divided into a plurality of areas to be achieved and which can provide an image being excellent in a viewing angle characteristics.
In view of the above, it is an object of the present invention to provide a multi-domain liquid crystal display device capable of making more reliable the effects obtained by the technology disclosed in the Japanese Patent Application No. Hei11-180615, of solving problems associated with conventional technologies including increased complicated processes such as microfabrication processes required for fabricating a common electrode and necessity for highly sophisticated laminating technology and of providing sharp contrast and excellent viewing angle characteristics.
According to a first aspect of the present invention, there is provided a multi-domain liquid crystal display device including:
a liquid crystal held and put between a pair of substrates;
two or more gate bus lines formed on one of the substrates and extending horizontally;
two or more drain bus lines extending vertically;
a plurality of pixels each being formed corresponding to each point of intersection of the gate bus lines and the drain bus lines and being arranged in a matrix form; and
whereby each pixel described above is provided with a switching device to select a desired pixel, with a pixel electrode and with a control electrode operating to form two or more orientation areas within one pixel by generating a skew electric field acting on the liquid crystal and wherein the control electrode is connected to one of terminals of the switching device and a coupling capacitor is connected between the pixel electrode and the control electrode and wherein a signal voltage is applied from the corresponding drain bus line through the corresponding switching device at a time of selecting the corresponding gate line and a partial voltage of the signal voltage is applied to the pixel electrode through the coupling capacitor.
In the foregoing, a preferable mode is one wherein the pixel electrode is formed under the control electrode and an interlayer dielectric is formed between the pixel electrode and the control electrode.
Also, a preferable mode is one wherein an aperture section is formed on the pixel electrode.
Also, a preferable mode is one wherein the control electrode controls an electric field acting on an orientation state of the liquid crystal through the aperture section.
Also, a preferable mode is one that wherein includes a common capacitor line to add capacitance to the pixel electrode.
Also, a preferable mode is one wherein the common capacitor line is disposed at the aperture section.
Also, a preferable mode is one that wherein includes a coupling capacitor having predetermined capacitance between the pixel electrode and the common capacitor line.
Also, a preferable mode is one wherein a part of the control electrode is composed of a transparent electrode.
Also, a preferable mode is one wherein the control electrode has quarter-wave plates at each side of a liquid crystal layer, which is formed in a manner that optic axes of the quarter-wave plates are intersected at right angles.
Also, a preferable mode is one wherein quarter-wave plates are provided on both sides of the liquid crystal and the quarter-wave plates are placed in a manner that their optic axes are intersected at right angles.
Also, a preferable mode is one wherein the switching device is a TFT (Thin Film Transistor) having a bottom gate structure.
Also, a preferable mode is one wherein the switching device is a TFT having a top gate structure.
Also, a preferable mode is one wherein the interlayer dielectric is formed integrally with a passivation film of the TFT.
Also, a preferable mode is one wherein the control electrode is formed integrally with a source terminal of the TFT.
Also, a preferable mode is one wherein the aperture section is formed in a window-like shape.
Also, a preferable mode is one wherein the aperture section is formed in a manner that incisions are exercised from one side or both sides of the pixel electrode.
Also, a preferable mode is one that wherein includes a resistor device to discharge electric charges accumulated in the pixel electrode between the pixel electrode and control electrode.
Also, a preferable mode is one wherein the resistor device has a substantially finite resistance value.
Also, a preferable mode is one that wherein includes a resistor device having a substantially finite resistance value disposed between the pixel electrode and common capacitor line.
Also, a preferable mode is one wherein an operation mode of the liquid crystal is a TN (Twisted Nematic) mode in which the liquid crystal having positive dielectric constant anisotropy is orientated in a twisted manner.
Also, a preferable mode is one wherein the liquid crystal is spontaneously chiral.
Also, a preferable mode is one wherein the liquid crystal is unspontaneously chiral.
Also, a preferable mode is one wherein the operation mode of the liquid crystal is a homogeneous mode in which the liquid crystal having positive dielectric constant anisotropy is orientated in a uniform manner.
Also, a preferable mode is one wherein the operation mode of the liquid crystal is a VA (Vertical Alignment) mode in which the liquid crystal having negative dielectric constant anisotropy is orientated in a homeotropic direction.
Also, a preferable mode is one wherein the pixel electrode is constructed of two or more fine pixel electrodes each being a square in shape and the control electrode is disposed along one side of the square and remaining three sides constitute a part of the aperture section or end portion of the pixel electrode.
Also, a preferable mode is one wherein the pixel electrode is constructed of two or more fine pixel electrodes each being a square in shape and the control electrode is disposed along two sides of the square and remaining two sides constitute a part of the aperture section or end portion of the pixel electrode.
Also, a preferable mode is one wherein the square is almost regular square.
Also, a preferable mode is one wherein the pixel electrode is constructed of two or more fine pixel electrodes each being a triangle in shape and the control electrode is disposed along two sides of the triangle and remaining one side constitutes a part of the aperture section or the pixel electrode end portion.
Also, a preferable mode is one wherein the pixel electrode is constructed of two or more fine pixel electrodes each being a triangle in shape and the control electrode is disposed along one side of the triangle and remaining two sides constitute a part of the aperture section or the pixel electrode end portion.
Also, a preferable mode is one wherein the pixel electrode is constructed of two or more fine pixel electrodes each being a pentagon in shape and the control electrode is disposed along two sides of the pentagon and remaining three sides constitute a part of the aperture section or the pixel electrode end portion.
Also, a preferable mode is one wherein the pixel electrode is constructed of a plurality of fine pixel electrodes and the fine pixel electrodes are constructed by a combination of more than two kinds of the fine pixel electrodes stated above.
Also, a preferable mode is one wherein a ratio of the control electrode voltage applied to the control electrode, relative to a voltage of the common electrode, to a pixel electrode voltage applied to the pixel electrode, relative to a voltage of the common electrode, is set to 1.1 to 1.4.
Also, a preferable mode is one wherein a ratio of the control electrode voltage to the pixel electrode voltage is set to 1.2 to 1.4.
Also, a preferable mode is one wherein a ratio of the control electrode voltage to the pixel electrode voltage is set to about 1.3.
Also, a preferable mode is one wherein the minute orientation area in which the liquid crystal is orientated in a uniform manner is about 20xc2x7m2 or less in size.
Also, a preferable mode is one wherein the minute orientation area in which the liquid crystal is orientated in a uniform manner is about 40xc2x7m2 or less in size.
Also, a preferable mode is one wherein the minute orientation area in which the liquid crystal is orientated in a uniform manner is 20xc2x7m2 to 40xc2x7m2 in size.
Also, a preferable mode is one wherein at least a part of the coupling capacitor is constructed by having a coupling capacitor terminal electrically connected to either of the pixel electrode or the control electrode overlain by other electrode to which the coupling capacitor terminal is not connected, with a gate insulating film interposed between them.
Also, a preferable mode is one wherein at least a part of the coupling capacitor is constructed by having an additional capacitor terminal electrically connected to either of the pixel electrode or common capacitor line overlain by other electrode to which the additional capacitor terminal is not connected, with a passivation film interposed between them.
Also, a preferable mode is one wherein at least a part of the coupling capacitor is constructed by having an additional capacitor terminal electrically connected to either of the pixel electrode or common capacitor line overlain by other electrode to which the additional capacitor terminal is not connected, with a gate insulating film interposed between them.
Also, a preferable mode is one that wherein includes a discharging device to discharge an electric charge accumulated at the pixel electrode mounted on the gate bus line corresponding to a front stage of an arbitrary pixel.
Furthermore, a preferable mode is one wherein the passivation film is removed from the area corresponding to the aperture section, out of aperture sections formed on the pixel electrode, on which a control electric field generated by the control electrode disposed in the lower layer acts.
According to a second aspect of the present invention, there is provided a multi-domain liquid crystal display device including:
a liquid crystal put between a pair of substrates;
a plurality of pixels disposed in a matrix form on one of the substrates;
a switching device mounted in each of the pixels;
a pixel electrode mounted in each of the pixels;
a control electrode mounted in each of the pixels and operated to generate skew electric fields acting on the liquid crystal to form two or more orientation areas in one pixel; and
whereby the control electrode is connected to one terminal of the switching device and wherein a coupling capacitor is connected between the pixel electrode and the control electrode and wherein a signal voltage is applied through the corresponding switching device to the control electrode and a partial voltage of the signal voltage is applied through the coupling capacitor to the pixel electrode.